diff --git a/internal/db/db.go b/internal/db/db.go
index fd34640..018add9 100644
--- a/internal/db/db.go
+++ b/internal/db/db.go
@@ -452,6 +452,11 @@ func runMigrations(d *sql.DB) error {
 		// the buyer when the target survives (the unseal), so piling on carries the
 		// same exposure the original purchase does.
 		`ALTER TABLE mischief_contracts ADD COLUMN escalated_by TEXT`,
+		// Pete games — a caller-supplied idempotency key for money moves that
+		// arrive over a retrying wire rather than a Matrix message. A Matrix
+		// message arrives once; a poll loop whose ack is lost on the wire will
+		// retry, and without this the player pays twice. See euro.DebitIdem.
+		`ALTER TABLE euro_transactions ADD COLUMN external_id TEXT`,
 	}
 	for _, stmt := range columnMigrations {
 		if _, err := d.Exec(stmt); err != nil {
@@ -468,6 +473,18 @@ func runMigrations(d *sql.DB) error {
 			return fmt.Errorf("migration %q: %w", stmt, err)
 		}
 	}
+
+	// Indexes over columns the column migrations above just added. These can't
+	// live in the schema block, which runs before those columns exist.
+	indexMigrations := []string{
+		`CREATE UNIQUE INDEX IF NOT EXISTS idx_euro_tx_external
+		   ON euro_transactions(external_id) WHERE external_id IS NOT NULL`,
+	}
+	for _, stmt := range indexMigrations {
+		if _, err := d.Exec(stmt); err != nil {
+			return fmt.Errorf("index migration %q: %w", stmt, err)
+		}
+	}
 	return nil
 }
 
diff --git a/internal/plugin/euro.go b/internal/plugin/euro.go
index 455b55b..e7a4772 100644
--- a/internal/plugin/euro.go
+++ b/internal/plugin/euro.go
@@ -1,8 +1,11 @@
 package plugin
 
 import (
+	"database/sql"
+	"errors"
 	"fmt"
 	"log/slog"
+	"math"
 	"os"
 	"regexp"
 	"strings"
@@ -413,6 +416,131 @@ func (p *EuroPlugin) GetBalance(userID id.UserID) float64 {
 	return balance
 }
 
+// ---------------------------------------------------------------------------
+// Idempotent money moves
+//
+// Every caller of Credit/Debit above is a Matrix message, which arrives exactly
+// once. Money that moves over a retrying wire — the Pete games escrow poll loop
+// — has no such guarantee: a claim that succeeds but whose ack is lost gets
+// retried, and the player pays twice. CreditIdem/DebitIdem take an externalID
+// (the escrow GUID), do the balance mutation and the transaction log in one
+// tx, and lean on idx_euro_tx_external so a replay is a no-op rather than a
+// second debit.
+//
+// Anything web-initiated goes through these. Nothing else should.
+// ---------------------------------------------------------------------------
+
+// DebitIdem subtracts euros exactly once for a given externalID. Returns ok=false
+// only when the debit would breach the debt limit; a replay of an already-applied
+// externalID returns ok=true without moving money again. balanceAfter is the
+// balance as of the decision, and is meaningless when ok is false.
+func (p *EuroPlugin) DebitIdem(userID id.UserID, amount float64, reason, externalID string) (ok bool, balanceAfter float64, err error) {
+	if amount <= 0 {
+		return false, 0, fmt.Errorf("euro: DebitIdem non-positive amount %v", amount)
+	}
+	if externalID == "" {
+		return false, 0, fmt.Errorf("euro: DebitIdem requires an external id")
+	}
+	debtLimit := envFloat("BLACKJACK_DEBT_LIMIT", 1000)
+	return p.applyIdem(userID, -amount, reason, externalID, -debtLimit)
+}
+
+// CreditIdem adds euros exactly once for a given externalID. A replay is a no-op
+// that still reports ok.
+func (p *EuroPlugin) CreditIdem(userID id.UserID, amount float64, reason, externalID string) (ok bool, balanceAfter float64, err error) {
+	if amount <= 0 {
+		return false, 0, fmt.Errorf("euro: CreditIdem non-positive amount %v", amount)
+	}
+	if externalID == "" {
+		return false, 0, fmt.Errorf("euro: CreditIdem requires an external id")
+	}
+	// A credit has no floor to breach — math.Inf would do, but the balance can
+	// never land below the current one, so any sentinel below it works.
+	return p.applyIdem(userID, amount, reason, externalID, math.Inf(-1))
+}
+
+// applyIdem is the shared body: signed delta, applied once, never letting the
+// balance fall below floor. Balance mutation and transaction log commit together
+// or not at all.
+func (p *EuroPlugin) applyIdem(userID id.UserID, delta float64, reason, externalID string, floor float64) (bool, float64, error) {
+	// Outside the tx: db is capped at a single connection, so a nested Exec on
+	// the pool would deadlock against our own open transaction.
+	p.ensureBalance(userID)
+
+	d := db.Get()
+	tx, err := d.Begin()
+	if err != nil {
+		return false, 0, fmt.Errorf("euro: begin: %w", err)
+	}
+	defer tx.Rollback() //nolint:errcheck // no-op after a successful Commit
+
+	// Already applied? Report the current balance and move on.
+	var prior int
+	switch err := tx.QueryRow(
+		`SELECT 1 FROM euro_transactions WHERE external_id = ?`, externalID,
+	).Scan(&prior); {
+	case err == nil:
+		bal, err := txBalance(tx, userID)
+		if err != nil {
+			return false, 0, err
+		}
+		slog.Info("euro: idempotent replay ignored", "user", userID, "external_id", externalID, "reason", reason)
+		return true, bal, nil
+	case errors.Is(err, sql.ErrNoRows):
+		// Not applied yet — carry on.
+	default:
+		return false, 0, fmt.Errorf("euro: idem lookup: %w", err)
+	}
+
+	res, err := tx.Exec(
+		`UPDATE euro_balances SET balance = balance + ?, updated_at = CURRENT_TIMESTAMP
+		 WHERE user_id = ? AND (balance + ?) >= ?`,
+		delta, string(userID), delta, floor,
+	)
+	if err != nil {
+		return false, 0, fmt.Errorf("euro: idem balance update: %w", err)
+	}
+	if affected, _ := res.RowsAffected(); affected == 0 {
+		return false, 0, nil // would breach the floor
+	}
+
+	if _, err := tx.Exec(
+		`INSERT INTO euro_transactions (user_id, amount, reason, external_id) VALUES (?, ?, ?, ?)`,
+		string(userID), delta, reason, externalID,
+	); err != nil {
+		// A racing caller won with the same externalID. The unique index is the
+		// backstop the SELECT above can't be: roll back our half-applied delta
+		// and report theirs.
+		if strings.Contains(err.Error(), "UNIQUE constraint failed") {
+			if rbErr := tx.Rollback(); rbErr != nil {
+				return false, 0, fmt.Errorf("euro: rollback after idem race: %w", rbErr)
+			}
+			slog.Info("euro: idempotent race lost, delta rolled back", "user", userID, "external_id", externalID)
+			return true, p.GetBalance(userID), nil
+		}
+		return false, 0, fmt.Errorf("euro: idem tx log: %w", err)
+	}
+
+	bal, err := txBalance(tx, userID)
+	if err != nil {
+		return false, 0, err
+	}
+	if err := tx.Commit(); err != nil {
+		return false, 0, fmt.Errorf("euro: commit: %w", err)
+	}
+	return true, bal, nil
+}
+
+func txBalance(tx *sql.Tx, userID id.UserID) (float64, error) {
+	var bal float64
+	if err := tx.QueryRow(
+		`SELECT balance FROM euro_balances WHERE user_id = ?`, string(userID),
+	).Scan(&bal); err != nil {
+		return 0, fmt.Errorf("euro: read balance: %w", err)
+	}
+	return bal, nil
+}
+
 func (p *EuroPlugin) logTransaction(userID id.UserID, amount float64, reason string) {
 	db.Exec("euro: log transaction",
 		"INSERT INTO euro_transactions (user_id, amount, reason) VALUES (?, ?, ?)",
diff --git a/internal/plugin/euro_idem_test.go b/internal/plugin/euro_idem_test.go
new file mode 100644
index 0000000..b3961db
--- /dev/null
+++ b/internal/plugin/euro_idem_test.go
@@ -0,0 +1,174 @@
+package plugin
+
+import (
+	"sync"
+	"testing"
+
+	"gogobee/internal/db"
+
+	"maunium.net/go/mautrix/id"
+)
+
+func newEuroTestDB(t *testing.T) *EuroPlugin {
+	t.Helper()
+	dir := t.TempDir()
+	db.Close()
+	if err := db.Init(dir); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(db.Close)
+	return NewEuroPlugin(nil)
+}
+
+func seedBalance(t *testing.T, p *EuroPlugin, user id.UserID, amount float64) {
+	t.Helper()
+	p.ensureBalance(user)
+	if _, err := db.Get().Exec(
+		`UPDATE euro_balances SET balance = ? WHERE user_id = ?`, amount, string(user),
+	); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestDebitIdem_ReplaySameGUIDDebitsOnce(t *testing.T) {
+	p := newEuroTestDB(t)
+	user := id.UserID("@replay:test.invalid")
+	seedBalance(t, p, user, 500)
+
+	for i := 0; i < 3; i++ {
+		ok, bal, err := p.DebitIdem(user, 100, "games_buyin", "guid-abc")
+		if err != nil {
+			t.Fatalf("attempt %d: %v", i, err)
+		}
+		if !ok {
+			t.Fatalf("attempt %d: debit rejected", i)
+		}
+		if bal != 400 {
+			t.Fatalf("attempt %d: balance after = %v, want 400", i, bal)
+		}
+	}
+
+	if got := p.GetBalance(user); got != 400 {
+		t.Fatalf("final balance = %v, want 400 (replay debited more than once)", got)
+	}
+
+	var n int
+	if err := db.Get().QueryRow(
+		`SELECT COUNT(*) FROM euro_transactions WHERE external_id = 'guid-abc'`,
+	).Scan(&n); err != nil {
+		t.Fatal(err)
+	}
+	if n != 1 {
+		t.Fatalf("logged %d transactions for one guid, want 1", n)
+	}
+}
+
+func TestCreditIdem_ReplaySameGUIDCreditsOnce(t *testing.T) {
+	p := newEuroTestDB(t)
+	user := id.UserID("@cashout:test.invalid")
+	seedBalance(t, p, user, 0)
+
+	for i := 0; i < 3; i++ {
+		ok, bal, err := p.CreditIdem(user, 250, "games_cashout", "guid-xyz")
+		if err != nil {
+			t.Fatalf("attempt %d: %v", i, err)
+		}
+		if !ok || bal != 250 {
+			t.Fatalf("attempt %d: ok=%v balance=%v, want true/250", i, ok, bal)
+		}
+	}
+
+	if got := p.GetBalance(user); got != 250 {
+		t.Fatalf("final balance = %v, want 250", got)
+	}
+}
+
+func TestDebitIdem_DistinctGUIDsBothApply(t *testing.T) {
+	p := newEuroTestDB(t)
+	user := id.UserID("@distinct:test.invalid")
+	seedBalance(t, p, user, 500)
+
+	for _, guid := range []string{"guid-1", "guid-2"} {
+		if ok, _, err := p.DebitIdem(user, 100, "games_buyin", guid); err != nil || !ok {
+			t.Fatalf("%s: ok=%v err=%v", guid, ok, err)
+		}
+	}
+	if got := p.GetBalance(user); got != 300 {
+		t.Fatalf("balance = %v, want 300", got)
+	}
+}
+
+func TestDebitIdem_RespectsDebtLimitAndLogsNothing(t *testing.T) {
+	t.Setenv("BLACKJACK_DEBT_LIMIT", "1000")
+	p := newEuroTestDB(t)
+	user := id.UserID("@broke:test.invalid")
+	seedBalance(t, p, user, 0)
+
+	ok, _, err := p.DebitIdem(user, 1500, "games_buyin", "guid-broke")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if ok {
+		t.Fatal("debit past the debt limit was accepted")
+	}
+	if got := p.GetBalance(user); got != 0 {
+		t.Fatalf("balance = %v, want 0 — a rejected debit moved money", got)
+	}
+
+	// A rejection must leave no trace, so the same guid can be retried later
+	// (e.g. once the player has earned their way back above the limit).
+	var n int
+	if err := db.Get().QueryRow(
+		`SELECT COUNT(*) FROM euro_transactions WHERE external_id = 'guid-broke'`,
+	).Scan(&n); err != nil {
+		t.Fatal(err)
+	}
+	if n != 0 {
+		t.Fatalf("rejected debit logged %d transactions, want 0", n)
+	}
+}
+
+// The SELECT-then-UPDATE in applyIdem is not by itself a guard against two
+// concurrent claims of the same guid; the unique index is. Hammer it.
+func TestDebitIdem_ConcurrentSameGUIDDebitsOnce(t *testing.T) {
+	p := newEuroTestDB(t)
+	user := id.UserID("@race:test.invalid")
+	seedBalance(t, p, user, 1000)
+
+	const racers = 8
+	var wg sync.WaitGroup
+	errs := make([]error, racers)
+	for i := 0; i < racers; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			_, _, errs[i] = p.DebitIdem(user, 100, "games_buyin", "guid-race")
+		}(i)
+	}
+	wg.Wait()
+
+	for i, err := range errs {
+		if err != nil {
+			t.Fatalf("racer %d: %v", i, err)
+		}
+	}
+	if got := p.GetBalance(user); got != 900 {
+		t.Fatalf("balance = %v, want 900 — %d concurrent claims of one guid double-debited", got, racers)
+	}
+}
+
+func TestIdem_RejectsEmptyExternalID(t *testing.T) {
+	p := newEuroTestDB(t)
+	user := id.UserID("@noguid:test.invalid")
+	seedBalance(t, p, user, 500)
+
+	if _, _, err := p.DebitIdem(user, 10, "games_buyin", ""); err == nil {
+		t.Fatal("DebitIdem accepted an empty external id")
+	}
+	if _, _, err := p.CreditIdem(user, 10, "games_cashout", ""); err == nil {
+		t.Fatal("CreditIdem accepted an empty external id")
+	}
+	if got := p.GetBalance(user); got != 500 {
+		t.Fatalf("balance = %v, want 500", got)
+	}
+}